Known liquid crystal compounds involve cholesteric liquid crystal compounds and nematic ones. Cholesteric liquid crystals have various optical properties based on a helical structure, for example, optical rotatory power, selective scattering of light, circular dichroism. These phenomena have been widely applied to a number of purposes wherein a color change of cholesteric liquid crystals caused by the application of voltage, a change in temperature or the adsorption of a gas is utilized. As particular examples of the application of cholesteric liquid crystals, there may be mentioned liquid crystal thermometers, pressure sensors, infrared light meters and microwave meter detectors. Many cholesterol compounds show cholesteric liquid crystal property. In addition, cholesteric liquid crystals can be obtained by dissolving an optically active substance having an optical rotatory power in nematic liquid crystals or by introducing an asymmetric carbon atom into a molecular structure of nematic liquid crystals. These liquid crystals, which are called chiral nematic liquid crystals, would not essentially differ from cholesteric liquid crystals of cholesterol compounds. However the former ones are superior to the latter ones in chemical and optical stability and thus have been employed instead of cholesterol liquid crystals.
Examples of such chiral nematic liquid crystals include N-(4-ethoxybenzylidene)-4-2-methylbutyl)aniline and 4-(2-methylbutyl)benzoic acid-4'-n-hexyloxyphenyl ester.
A substance to be used as chiral nematic liquid crystals should be highly stable to moisture, air, light and heat and sustain a cholesteric phase over a wide range of temperature at which a display element is to be used in general.
Although compounds or compositions mainly employed in liquid crystal display devices at present comprise these nematic liquid crystals, they have a serious disadvantage, namely, a low response speed of a several millisecond order. Thus it is considered that the enlargement of these display devices might be restricted thereby.
In order to improve this disadvantage of known liquid crystal display devices, N. A. Clark and S. T. Largerwall proposed to use liquid crystals having a bistability, as described in JP-A-56-107216. (The term "JP-A" herein used means an "unexamined published Japanese patent application".) These liquid crystals having a bistability are called ferroelectric liquid crystals and attract public attention since they can give high speed response and memory properties. Recently, it has been frequently attempted to put these ferroelectric liquid crystals into practical use. Thus it has been urgently required to develop practically available ferroelectric liquid crystal materials.
Generally speaking, the ferroelectricity is actualized with a compound having an optically active moiety in a smectic phase where the long axis of the molecule shows an orientation tilting against the normal direction of the layer made of the compound. A chiral smectic phase (hereafter referred to as S.sub.c *) is particularly advantageous from a practical viewpoint since the driving voltage thereof is relatively low.
Thus ferroelectric liquid crystals exhibit an extremely high response due to spontaneous polarization, can express a bistable state of high memory properties, has an excellent viewing angle, and are suitable for a display material of a large capacity and a large picture.
A known example of such ferroelectric liquid crystal compounds is (S)-2-methylbutyl 4-(4-decyloxybenzylideneamino)cinnomate (hereafter referred to as DOBAMBC) synthesized by R. B. Meyer et al. as described in J. Physique, 36 L-69 (1975).
This DOBAMBC contains a Schiff base in its structure, which causes a problem in chemical stability. Therefore there has been attempted to find out ferroelectric liquid crystal compounds which are physically and chemically stable. Now the main current of these studies goes toward esters such as (S)-2-methylbutyl 4-(4-n-alkoxybenzoyloxy)benzoate. However, these esters would show either no S.sub.c * phase or an S.sub.c * phase, if any, within a considerably narrow temperature range. Furthermore, they are monotropic liquid crystals whose phase system achieved by heating the liquid crystals to a certain temperature differs from that achieved by cooling the same to the same temperature. Accordingly, there are only a few compounds which are practically applicable, as described in Liquid Crystals and Ordered Fluids, 4 (1984).
Particular examples of sources for providing optically active moieties of known ferroelectric liquid crystal compounds involve optically active alcohols such as 2-methylbutanol and 3-methylpentanol. However these alcohols are expensive, which brings about a problem in production cost. JP-A-62-89648 proposes to use an amino acid, which can be more easily obtained at a lower cost, as an optically active material. According to this report, however, the product thus obtained would show only a smectic A phase at a high temperature and therefore is not practical.